Explanation:
The RECOVER DATABASE command can only be executed when the database is in MOUNT state. Note that table space or data file recovery is done while the database is online or offline. However, if you recover while the database is online, you need to make sure that the corresponding data files and table spaces are offline. Database recovery always requires you to keep the database offline (database mounted) while all the data files that you need to recover should be online.
Reference:
Oracle 9i Net Services Administrator’s Guide
Oracle 9i Net Services Reference Guide
Oracle 9i Recovery Manager Reference
Oracle 9i Backup and Recovery Concepts

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